/* @flow */

import type Router from "../index";
import { assert } from "./warn";
import { getStateKey, setStateKey } from "./push-state";

const positionStore = Object.create(null);

export function setupScroll() {
  // Fix for #1585 for Firefox
  window.history.replaceState({ key: getStateKey() }, "");
  window.addEventListener("popstate", (e) => {
    saveScrollPosition();
    if (e.state && e.state.key) {
      setStateKey(e.state.key);
    }
  });
}

export function handleScroll(
  router: Router,
  to: Route,
  from: Route,
  isPop: boolean
) {
  if (!router.app) {
    return;
  }

  const behavior = router.options.scrollBehavior;
  if (!behavior) {
    return;
  }

  if (process.env.NODE_ENV !== "production") {
    assert(typeof behavior === "function", `scrollBehavior must be a function`);
  }

  // wait until re-render finishes before scrolling
  router.app.$nextTick(() => {
    const position = getScrollPosition();
    const shouldScroll = behavior.call(
      router,
      to,
      from,
      isPop ? position : null
    );

    if (!shouldScroll) {
      return;
    }

    if (typeof shouldScroll.then === "function") {
      shouldScroll
        .then((shouldScroll) => {
          scrollToPosition((shouldScroll: any), position);
        })
        .catch((err) => {
          if (process.env.NODE_ENV !== "production") {
            assert(false, err.toString());
          }
        });
    } else {
      scrollToPosition(shouldScroll, position);
    }
  });
}

export function saveScrollPosition() {
  const key = getStateKey();
  if (key) {
    positionStore[key] = {
      x: window.pageXOffset,
      y: window.pageYOffset,
    };
  }
}

function getScrollPosition(): ?Object {
  const key = getStateKey();
  if (key) {
    return positionStore[key];
  }
}

function getElementPosition(el: Element, offset: Object): Object {
  const docEl: any = document.documentElement;
  const docRect = docEl.getBoundingClientRect();
  const elRect = el.getBoundingClientRect();
  return {
    x: elRect.left - docRect.left - offset.x,
    y: elRect.top - docRect.top - offset.y,
  };
}

function isValidPosition(obj: Object): boolean {
  return isNumber(obj.x) || isNumber(obj.y);
}

function normalizePosition(obj: Object): Object {
  return {
    x: isNumber(obj.x) ? obj.x : window.pageXOffset,
    y: isNumber(obj.y) ? obj.y : window.pageYOffset,
  };
}

function normalizeOffset(obj: Object): Object {
  return {
    x: isNumber(obj.x) ? obj.x : 0,
    y: isNumber(obj.y) ? obj.y : 0,
  };
}

function isNumber(v: any): boolean {
  return typeof v === "number";
}

function scrollToPosition(shouldScroll, position) {
  const isObject = typeof shouldScroll === "object";
  if (isObject && typeof shouldScroll.selector === "string") {
    const el = document.querySelector(shouldScroll.selector);
    if (el) {
      let offset =
        shouldScroll.offset && typeof shouldScroll.offset === "object"
          ? shouldScroll.offset
          : {};
      offset = normalizeOffset(offset);
      position = getElementPosition(el, offset);
    } else if (isValidPosition(shouldScroll)) {
      position = normalizePosition(shouldScroll);
    }
  } else if (isObject && isValidPosition(shouldScroll)) {
    position = normalizePosition(shouldScroll);
  }

  if (position) {
    window.scrollTo(position.x, position.y);
  }
}
